The invention relates generally to locating the position of a mobile communication unit in a wireless communication network and, more particularly, to uplink time of arrival measurements.
The ability to locate the position of a mobile communication unit operating in a wireless communication system provides many well known advantages. Exemplary uses of such position locating capability include security applications, emergency response applications, and travel guidance applications. Among several possible techniques for providing position locating capability, uplink time of arrival (TOA) techniques are attractive because they normally do not require any changes to the mobile communication units.
One example of an uplink time of arrival approach will now be described with respect to the Global System for Mobile Communication (GSM), which is exemplary of a wireless communication system in which uplink time of arrival techniques are applicable. When an external application (or the GSM network itself) decides to locate the position of a mobile unit (also referred to as mobile station), a Mobile Location Center forces (via a base station controller) the mobile unit to perform a conventional intracell handover, whereupon the mobile unit transmits up to 70 uplink access bursts, one burst per TDMA frame (i.e., one burst every 8 time slots). The mobile unit transmits the access bursts in an attempt to comply with the intracell handover command.
The Mobile Location Center (MLC) orders a number of TOA Measurement Units (TMUs) to capture the access bursts and measure the time of arrival of each burst at each TMU. The TMUs then provide the MLC with their time of arrival measurements and reliability estimates for these measurements. In order to compute the position of the mobile station, the MLC uses the time of arrival values and corresponding reliability parameters, the geographic location coordinates of the TMUs, and information regarding time differences among the respective internal time bases of the TMUs. For example, each TMU can be provided with an absolute time reference (e.g., a Global Positioning System (GPS) clock), in which case the TMUs are all synchronized together, so that relative time differences among the TMUs are not a factor in the MLC""s calculation of the position of the mobile station.
However, if the TMUs do not include an absolute time reference, then the relative differences among their respective local time references can be determined, for example, by having each TMU measure the time of arrival of an uplink burst from a stationary reference mobile station positioned at a known location in the network. The time of arrival information for the reference mobile station is then transmitted from the TMUs to the MLC. The MLC can use these time of arrival measurements to compute the relative differences in the timing references of the respective TMUs, also referred to as inter-TMU time differences (ITTDs).
More specifically, because the MLC knows the location of the reference mobile station and also knows the locations of the TMUs, the MLC can easily calculate the expected difference (in absolute time) between the time of arrival of the burst at a first TMU and the time of arrival of the burst at a second TMU. Then, when the MLC receives the time of arrival information as actually observed at the first and second TMUs, it can compare the difference between observed arrival times to the expected difference as previously calculated. By this comparison, the actual time difference between the local time references of the first and second TMUs (the ITTD of the first and second TMUs) can be readily determined. Time of arrival measurements on the reference mobile station can be made periodically by the TMUs, and provided to the MLC for use in determining the ITTDs, so that MLC can maintain an updated record of the ITTDs.
In addition to the technique described above, other conventional techniques are also available for determining the ITTDs.
Because the MLC knows the ITTDs (or alternatively knows that the TMUs are all synchronized by a GPS system), it can calculate an estimate of the position of a given mobile station from the time of arrival information provided by the TMUs, using conventional Time Difference of Arrival (TDOA) techniques.
One problem with the above-described uplink time of arrival techniques is that the TMUs do not know when they should expect, or begin to monitor for, the access bursts from the mobile station. This has the following disadvantages. The sensitivity of conventional time of arrival measurement algorithms decreases as the uncertainty in the a priori knowledge of the arrival time increases. This is due to the fact that more noise and interference is received if the receiver does not know when the xe2x80x9cmeaningfulxe2x80x9d data is coming. Also, the TMU must monitor for a long time in order to be sure that it captures the desired bursts. Thus, the utilization efficiency of the TMU hardware is disadvantageously degraded.
Furthermore, in a frequency hopping system such as GSM, the access bursts are transmitted on a frequency hopped channel. In this situation, not only does the TMU not know when to begin monitoring for the access bursts, but it also does not know which frequency it should monitor. Thus, not only is an undesirably long monitoring time possible, but the TMU would also be required to include a receiver for each frequency in the hop sequence so that all possible frequencies could be monitored.
It is therefore desirable to provide time of arrival measuring devices with information indicative of when the access bursts can be expected to arrive, and which frequency will be used to transmit the access bursts. According to the present invention, such information is provided to time of arrival measuring devices, thereby avoiding the aforementioned problems associated with conventional time of arrival techniques.
Also, for positioning purposes, time of arrival measurements are sometimes performed on radio links having a signal-to-noise and interference ratio (SNIR) far below zero. Especially indoors and in rural environments, sensitivity can become a problem. Also, the TMUs that will perform the time of arrival measurements must be selected before the actual measurements are performed, when no information on SNIR is available. Thus, it is unavoidable that the radio links between at least some of the selected TMUs and the selected mobile unit will have a very poor SNR. Prediction simulators indicate that every dB that can be won in the measurement is valuable. This is especially true with respect to TMUs implemented without antenna diversity, in which case several dB are lost. This of course degrades the system performance significantly.
It is therefore desirable to provide the TMUs with enhanced sensitivity to the signaling from the mobile unit. This is achieved according to the present invention by providing the TMUs with information from which the TMUs can determine in advance certain encrypted bits in the uplink access burst that will be transmitted by the mobile unit.